Solution mechanisms of H 2O in depolymerized peralkaline melts

Abstract

Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°–1300°C) at high pressure (0.8–2.0 GPa) have been examined in peralkaline melts in the system Na 2O-SiO 2-H 2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂X H2O, decreases with increasing total water content. At low water contents, the influence of H 2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q 4-species to form Q 3 and Q 2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q 3-species react with water to form Q 2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H- 29Si cross polarization NMR data.